The speaker is used to convert the audio signal to the sound. The low frequency part of the audio signal may not be successfully converted by the speaker with general or poor performance, and that is, the speaker with general or poor performance cannot play the low frequency sound very well. Thus, the audio processing circuit in the speaker may filter out the low frequency part of the audio signal, but this causes the sound played by the speaker distort (i.e. the low frequency the sound cannot be listened by the user).
Referring to FIG. 1, FIG. 1 is a schematic diagram showing the psychoacoustic illusion. In FIG. 1, the actual signals of frequencies 2f, 3f and 4f are physically present, and the perceived bass signal of the lowest fundamental frequency f is virtually listened by the user, even if the perceived bass signal of the lowest fundamental frequency f is not physical present.
The psychoacoustic illusion can be used to generate the perceived bass signal for the speaker with the general or poor performance. The nonlinear device can be used to generate harmonics in series of the lowest fundamental frequency associated with the perceived bass signal. For example, the full wave rectifier can be used to generate the even harmonics, the soft clipping circuit can be used to generate the odd harmonics, and the circuit with the full wave rectifier and the integrator can be used to generate both of the even and odd harmonics.
Referring to FIG. 2, FIG. 2 is a schematic diagram showing a conventional virtual bass generating circuit. The conventional virtual bass generating circuit is used in the speaker. The conventional virtual bass generating circuit comprises a first-order high pass filter 10, a first-order low pass filter 12, an absolute value circuit 14, a second-order high pass filter 16, amplifiers 18, 28, 30, an adder 20, a second-order low pass filter 22, a clip circuit 24 and subtractors 26, 32.
An input end of the conventional virtual bass generating circuit is electrically connected to the subtractor 32 and the first high pass filter 10. The first high pass filter 10 is electrically connected the first low pass filter 12. The first low pass filter 12 is electrically connected to the absolute value circuit 14, the clip circuit 24 and the amplifier 30. The clip circuit 24 and the amplifier 30 are electrically connected to the subtractor 26, and the subtractor 26 is electrically connected to the amplifier 28. The absolute value circuit 14 is electrically connected to the second-order high pass filter 16, and the second-order high pass filter 16 is electrically connected to the amplifier 18. The amplifiers 18 and 28 are electrically connected to the adder 20, and the adder 20 is electrically connected to the second-order low pass filter 22. The second-order low pass filter 22 is electrically connected to the subtractor 32, and the subtractor 32 is electrically connected an output end of the conventional virtual bass generating circuit.
An audio signal is received by the input end of the conventional virtual bass generating circuit. The first high pass filter 10 and the first low pass filter 12 respectively filter out the low frequency part and the high frequency part of the audio signal to generate a first filtered audio signal. The absolute value circuit 14 obtains the absolute part of the first filtered audio signal, and functions as the full wave rectifier, so as to output the rectified audio signal. The second-order high pass filter 16 filters out the low frequency part of the rectified audio signal to generate a second filtered audio signal. The amplifier 18 amplifies the second filtered audio signal with the gain G1 to generate a first amplified audio signal.
The clip circuit 24 is used to clip the first filtered audio signal to generate a clipped audio signal, and the amplifier 30 amplifies the first filtered audio signal with the gain G3 to generate a third amplified audio signal. The subtractor 26 subtracts the third amplified audio signal from the clipped audio signal to generate a first subtracting audio signal. The amplifier 28 amplifies the first subtracting audio signal with the gain G2 to generate a second amplified audio signal.
The adder 20 adds the first amplified audio signal and the second amplified audio signal to generate an adding audio signal. The second low pass filter 22 filters out the high frequency part of the adding audio signal to generate a third filtered audio signal. The subtractor 32 subtracts the third filtered audio signal from the audio signal to generate a second subtracting audio signal. The conventional virtual bass generating circuit outputs the second subtracting audio signal which is played by the speaker. It is noted that that the first and second high pass filters 10 and 16 can prevent the direct current part of the audio signal from being played by the speaker. However, the conventional virtual bass generating circuit still needs four filters (i.e. 10, 12, 16 and 22), thus result high hardware cost.